Can anyone tell me the size auto tire that replaces a 180/70-15 motorcycle tire on a Vulcan 900?  Has anyone used auto tires on their bike and how did they perform?

Auto tires are designed completely different that bike tires. Bike tires have 3 different types of rubber in the different contact zones maintain traction. Auto tires are traditionally too soft and don't have the right contact with the road for motorcycle use. The only person I know that actually tried it only made it a couple hundred miles before failure.

  Car tires are not made for and not safe on motorcycles.  They are not designed to lean and have NO traction when leaning in turns.

I have seen them used on a few customs, but never on any bike that was intended to actually be ridden like a motorcycle.

This is on a bike with a Voyager kit---so it will not be ridden like a regular bike.

Maybe you should try a trike or Vulcan forum to see if anyone there has made this switch?
www.vulcanforums.com /> www.triketalk.com/forums/

I have heard mixed reviews on the use of auto tires for motorcycles.There is a dedicated group of riders who maintain that using them is safe & feasible. I personally do not think that is a smart way to ride.Other than longevity there is no true advantage to using a car tire over a motorcycle tire.

I'm a little confused about what you are trying to do. Motorcycle rims are sized for motorcycle tires. In most cases,they cannot/ should not be replace with car tires. If the the tire you need is for a trike then the front tire still needs to be a motorcycle tire and the rear tires need to be fitted to the rims that come with the trike kit.

While we are on the subject of tires a little fact about tire looks.........I know when you wash your bike you want those tires looking sharp but never put Armor on your tires. I know someone who did (NO it was not me ) and his first curve at speed he found out why you don't put Armor on your tires.
I know to most this is a no brainier, but I guess that is why warning's are on products.
Be Safe
ND

 For some of you with this idea here is a few viedos on Youtube you may like to look at.  

https://www.youtube.com/watch?v=gZKhoFbL7Fo />
https://www.youtube.com/watch?v=PZ_d5IIdRZI />
https://www.youtube.com/watch?v=fkB0JnAPY0w

It's called (by some) riding the Darkside. Try this: http://mcdarksiders.forumotion.com/ /> or http://www.ridermagazine.com/browse-by-type/tires/tales-from-the-dark-side-putting-car-tires-on-motorcycles.htm/ />
I have to admit, my PanShovel chop had a 18" VW tire on the back. It took a little while to get used to the handling characteristics but not too long. I rode that bike like I stole it and she never let me down. So check out the websites and let us know what happens!